IU Health said it will send letters to 6,500 patients who were treated at Methodist, University, Arnett and Bloomington hospitals since 2012.

Letters have already been sent to another 1,800 patients who were treated at the Roudebush VA Medical Center, Franciscan Health, and Community Health.

No infections have been reported in Indiana, but hospitals are notifying patients because it can take up to four years for symptoms to appear. Infections have occurred in Michigan, Iowa and Minnesota.

Patients with symptoms like night sweats, weight-loss, fatigue, joint pain, fever, or loss of appetite should see their doctor right away.

The source of the infection is a specific machine used at hospitals during bypass procedures to keep the patient warm — called the Stöckert 3T Heater-Cooler Machine, made in Germany by LivaNova.

The machine is used by nearly every hospital that performs open-heart surgery in the United States, and experts believe many were contaminated at the manufacturing facility in Germany.

The infections are caused by a bacteria known as Non-Tuberculosis Mycobacterium (NTM). The risk of infection is less than 1%, but it can be deadly, according to the Centers for Disease Control (CDC).

The CDC warned that patients who had valves or prosthetic devices implanted have a higher risk of infection.

A year-long treatment with antibiotics can cure the infection, but without proper diagnosis it can be deadly. From 2010 to 2016, the FDA received 91 reports of patients who developed NTM infections related to heater-cooler devices, resulting in at least 12 deaths.